1. Field of Invention
The field of the currently claimed embodiments of this invention relates to Fourier domain optical coherence tomography systems (FD-OCT) and more particularly to methods of automatically calibrating FD-OCT systems.
2. Discussion of Related Art
Integrating optical coherence tomography (OCT) in handheld or robot-assisted surgical tools for microsurgery can potentially minimize damage to tissue and improve surgical outcomes. See the following for some examples of applications where methods according to some embodiments of the current invention can be utilized:                S. Han, M. V. Sarunic, J. Wu, M. Humayun, and C. Yang, “Handheld forward-imaging needle endoscope for ophthalmic optical coherence tomography inspection”, Journal of Biomedical Optics 13, 020505 (2008);        M. Balicki, J. Han, I. Iordachita, P. Gehlbach, J. Handa, J. U. Kang, R. Taylor, “Single Fiber Optical Coherence Tomography Microsurgical Instruments for Computer and Robot-Assisted Retinal Surgery”, Proceedings of the MICCAI Conference, London, pp. 108-115 (2009);        K. Zhang, W. Wang, J. Han and J. U. Kang, “A surface topology and motion compensation system for microsurgery guidance and intervention based on common-path optical coherence tomography,” IEEE Transactions on Biomedical Engineering, Vol. 56, pp. 2318-2311 (2009);        J. Han, M. Balicki, K. Zhang, X. Liu, J. Handa, R. Taylor, and J. U. Kang, “Common-path Fourier-domain Optical Coherence Tomography with a Fiber Optic Probe Integrated Into a Surgical Needle”; Proceedings of CLEO Conference (2009); and        Y. K. Tao, J. P. Ehlers, C. A. Toth, and J. A. Izatt, “Intraoperative spectral domain optical coherence tomography for vitreoretinal surgery,” Opt. Lett. 35, 3315-3317 (2010).        
Fourier Domain OCT (FD OCT), which offers significantly improved sensitivity and imaging speed compared to time-domain OCT (TD-OCT) (J. de Boer, B. Cense, B. Hyle Park, M. C. Pierce, G. J. Tearney, and B. E. Bouma, “Improved signal-to-noise ratio in spectral-domain compared with time-domain optical coherence tomography,” Opt. Lett. 28, 2067-2069 (2003); R. Leitgeb, C. Hitzenberger, and Adolf Fercher, “Performance of Fourier domain vs. time domain optical coherence tomography,” Opt. Express 11, 889-894 (2003); M. Choma, M. Sarunic, C. Yang, and J. Izatt, “Sensitivity advantage of swept source and Fourier domain optical coherence tomography,” Opt. Express 11, 2183-2189 (2003); X. Li, J. Han, X. Liu, and J. U. Kang, “Signal-to-noise ratio analysis of all-fiber common-path optical coherence tomography,” Appl. Opt. 47, 4833-4840 (2008); U. Sharma, N. M. Fried, and J. U. Kang, “All-fiber Fizeau optical coherence tomography: sensitivity optimization and system analysis,” IEEE J. Quantum Electron. 11799-805 (2005); X. Liu, X. Li, D. Kim, I. Ilev, and J. U. Kang, “Fiber-optic Fourier-domain common-path OCT,” Chin. Opt. Lett. 6, 899-901 (2008)), has been incorporated with robotic surgical tools for vitreoretinal surgery applications. For example, such systems can use real-time, tool-to-tissue range data derived from OCT images to actively enforce safety barriers, compensate for patient motion, or guide the surgeon to perform a pre-planned maneuver (M. Balicki, J. Han, I. Iordachita, P. Gehlbach, J. Handa, J. U. Kang, R. Taylor, “Single Fiber Optical Coherence Tomography Microsurgical Instruments for Computer and Robot-Assisted Retinal Surgery”, Proceedings of the MICCAI Conference, London, pp. 108-115 (2009); K. Zhang, W. Wang, J. Han and J. U. Kang, “A surface topology and motion compensation system for microsurgery guidance and intervention based on common-path optical coherence tomography,” IEEE Transactions on Biomedical Engineering, Vol. 56, pp. 2318-2311 (2009)). In such applications, it is critical for OCT to have high axial imaging resolution and precise depth ranging functionality. Further, safety, reliability, and ease of use are important factors in a demanding application like microsurgery, where imaging devices are exposed to extreme handling conditions, require frequent safety checks, and redundant monitoring during operation.
FD OCT has two subcategories: 1) spectral domain OCT and 2) swept-source OCT (A. F. Fercher, W. Drexler, C. K. Hitzenberger and T. Lasser, “Optical coherence tomography—principles and applications,” Rep. Prog. Phys. 66, 239-303 (2003); B. E. Bouma and G. J. Tearney, “Handbook of Optical Coherence Tomography”, Marcel Dekker, New York (2002)). In this specification, “FD OCT” specifically refers to spectral domain OCT, which uses a spectrometer to detect spectral interferograms. This is typically an inexpensive approach. To achieve high axial resolution and ranging accuracy, FD OCT requires not only a broadband light source and a well-designed spectrometer, but also an accurate spectral calibration to correctly reconstruct the sample's depth profile (A-scan). To obtain an A-scan from the spectral interferogram captured by the spectrometer, an inverse Fourier transformation is applied to the interference spectral data that is evenly spaced in wavenumber space (k-space). See, for example:                Y. Chen, B. Sun, T. Han, Y. Kong, C. Xu, P. Zhou, X. Li, S. Wang, Y. Zheng, L. Chen, “Densely folded spectral images of a CCD spectrometer working in the full 200-1000 nm wavelength range with high resolution”, Opt. Express 13, 10049-10054 (2005);        M. Mujat, B. H. Park, B. Cense, T. C. Chen, J. de Boer, “Autocalibration of spectral-domain optical coherence tomography spectrometers for in vivo quantitative retinal nerve fiber layer birefringence determination”, J Biomed Opt., 12(4):041205 (2007);        C. Ding, P. Bu, X. Wang, O. Sasakic, “A new spectral calibration method for Fourier domain optical coherence tomography”, Opt. Int. J. Light Electron. Opt. (2009);        E. Azimi, B. Liu, M. E. Brezinski., “Real-time and high-performance calibration method for high-speed swept-source optical coherence tomography,” J Biomed Opt., 15(1):016005 (2010);        J. Xi, L. Huo, J. Li, and X. Li, “Generic real-time uniform K-space sampling method for high-speed swept-Source optical coherence tomography,” Opt. Express 18, 9511-9517 (2010)        
However, in FD OCT, the spectra are detected by CCD or CMOS arrays, which usually do not guarantee a uniform sampling in k-space. Converting the data from pixel space to k-space depends on knowing the wavenumber at each pixel of the array detector, which is usually determined through a calibration process. Poor or imprecise calibration results in a point spread function (PSF) that has a depth-dependent broadening, analogous to pulse broadening induced by group velocity dispersion. Besides significantly degrading system performance in terms of the resolution and sensitivity, an inaccurate calibration also leads to erroneous depth ranging.
Calibration of a spectrometer in FD OCT can be achieved either by measuring the spectrum of an external calibrating light source with known spectral features (Y. Chen, B. Sun, T. Han, Y. Kong, C. Xu, P. Zhou, X. Li, S. Wang, Y. Zheng, L. Chen, “Densely folded spectral images of a CCD spectrometer working in the full 200-1000 nm wavelength range with high resolution”, Opt. Express 13, 10049-10054 (2005)) or by comparing spectral interferograms measured with the OCT spectrometer to measurements made by a well-calibrated commercial optical spectrum analyzer (OSA) (C. Ding, P. Bu, X. Wang, O. Sasakic, “A new spectral calibration method for Fourier domain optical coherence tomography”, Opt. Int. J. Light Electron. Opt. (2009)). However, these conventional calibration methods are time-consuming and require separate measurements and extra equipment. These factors make conventional calibrations inconvenient in a clinical setting. Moreover, the characteristics of a spectrometer will naturally change over time, due both to environmental effects such as temperature and vibration and to poor handling practices (M. Mujat, et al). Therefore, monitoring and recalibration of the spectrometer may be necessary for each FD OCT measurement session. Furthermore, when using OCT in imaging and servoing for image-guided, robot-assisted surgery, the refractive index of the medium might be unknown and thus will impose a challenge in accurately determining the distance between the probe and sample surfaces. Since OCT measures optical path length, which is the product of physical distance and the medium's refractive index, a wrong estimation of this physical distance can cause inaccuracies in imaging, targeting errors, and robot control instabilities. All are unacceptable for high-risk microsurgical applications. A simple and automatic OCT calibration protocol that addresses these issues is required.
M. Mujat et al reported an automatic spectrometer calibration, based on generating a perfect sinusoidal spectral modulation in k-space by inserting a thin glass slide into the optical path ((M. Mujat, et al). The algorithm used requires that the spectrum has a perfect sinusoidal modulation; otherwise, it is impossible to obtain the phase for calibration. Moreover, M. Mujat et al's calibration does not produce the values for wavenumber limits; therefore, the physical pixel spacing of the OCT A-scan is still unclear after calibration. The specular reflection of the inserted glass slide may occupy a large portion of the detector's dynamic range, thus may reduce the dynamic range usable for sample and reference signal; on the other hand, the reduction of power from the broadband source may reduce the system's sensitivity. In Iftimia et al's spectral calibration, they circumvented the abovementioned problems by inserting the glass slide into the reference arm with power attenuation (N. V. Iftimia, D. X. Hammer, R. D. Ferguson, M. Mujat, D. Vu, and A. A. Ferrante, “Dual-beam Fourier domain optical Doppler tomography of zebrafish,” Opt. Express 16, 13624-13636 (2008)). Therefore, there remains a need for improved methods and systems for calibrating FD OCT systems.